Positive active material for lithium-ion secondary battery, positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

ABSTRACT

A positive active material for a lithium-ion secondary battery includes a lithium composite oxide particle containing nickel atoms, manganese atoms, and fluorine atoms. The lithium composite oxide particle includes a particle center portion and a surface layer portion that is closer to a surface of the lithium composite oxide particle than the particle center portion is. A fluorine atom concentration Fc (at %) of the particle center portion measured by energy dispersive X-ray spectroscopy is lower than a fluorine atom concentration Fs (at %) of the surface layer portion.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-149291 filed onJul. 22, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positive active material for alithium-ion secondary battery, a positive electrode for a lithium-ionsecondary battery, and a lithium-ion secondary battery.

2. Description of Related Art

As a positive active material capable of providing a high energy densityof a lithium-ion secondary battery, a spinel lithium nickel manganesecomposite oxide having an operating potential of 4.5 V or higher on thebasis of metal lithium (hereinafter, a potential based on metal lithiummay be represented by “vs.Li/Li⁺”) is known. However, in the batteryusing the composite oxide as the positive active material, for example,when charging and discharging are repeated under the condition of apositive electrode potential of 4.5 V (vs.Li/Li⁺) or higher, metalelements (typically manganese) elute from the positive active material,and the durability thereof may be significantly degraded. As a techniquefor a countermeasure to this problem, for example, in Materials ResearchBulletin, 2008, Volume 43, Issue 12, Pages 3607-3613, enhancingdurability by substituting a portion of oxygen (O) atoms in the lithiumnickel manganese composite oxide with fluorine (F) atoms is described.

In a case of applying the technique to a battery in which both of a highenergy density and a high output density are required (for example, anin-vehicle battery), there is still room for improvement. In a batteryhaving a lithium nickel manganese composite oxide in which a portion ofoxygen atoms is replaced with fluorine atoms, binding of the anions (O,F) and the cations (Ni, Mn) of the lithium nickel manganese compositeoxide is increased due to the fluorine, and thus elution of metalelements from the composite oxide can be suppressed. This results in atendency to increase durability. However, in a case where the amount offluorine is small, when charging and discharging are repeated undersevere conditions (for example, under the conditions of charging untilthe potential of the positive electrode reaches 4.5 V (vs.Li/Li⁺) in anenvironment at a high temperature of 50° C. or higher), the effect ofenhancing durability cannot be sufficiently obtained, and a non-aqueouselectrolyte at the positive electrode is oxidized and decomposed,resulting in an increase in the amount of generated gas. On the otherhand, when the amount of fluorine is increased in consideration of theenhancement of durability and a reduction in the amount of generatedgas, the interaction between the fluorine and a charge carrier (Li ions)is increased, and thus the diffusibility (mobility) of Li ions in thecomposite oxide is degraded. As a result, the battery resistance isincreased, and particularly, input and output characteristics aredegraded during high-rate charging or discharging. That is, although theincrease in the amount of fluorine enhances durability or reduces thegas generation amount, there is a contradiction that the batteryresistance is increased.

SUMMARY OF THE INVENTION

The present invention provides a positive active material for alithium-ion secondary battery, a positive electrode for a lithium-ionsecondary battery, and a lithium-ion secondary battery.

A first aspect of the present invention is a positive active materialfor a lithium-ion secondary battery. The positive active materialincludes a lithium composite oxide particle containing nickel atoms,manganese atoms, and fluorine atoms. The lithium composite oxideparticle includes a particle center portion and a surface layer portionthat is closer to a surface of the lithium composite oxide particle thanthe particle center portion is. A fluorine atom concentration Fc (at %)of the particle center portion measured by energy dispersive X-rayspectroscopy is lower than a fluorine atom concentration Fs (at %) ofthe surface layer portion.

By relatively reducing the fluorine atom concentration Fc of theparticle center portion, Li diffusibility in the particle can beensured, and thus an increase in resistance can be suppressed. Inaddition, by relatively increasing the fluorine atom concentration Fs ofthe surface layer portion which is closer to the surface than theparticle center portion is, elution of metal elements (typicallymanganese) from the lithium nickel manganese composite oxide or theoxidative decomposition of the non-aqueous electrolyte can besuppressed. Therefore, according to the positive active material in thefirst aspect of the present invention, the fluorine atoms contained inthe positive electrode efficiently suppresses the elution of metalelements or the oxidative decomposition of the non-aqueous electrolyte.Thus, a lithium-ion secondary battery having both high durability andexcellent input and output characteristics can be obtained.

In the first aspect of the present invention, a portion of oxygen atomsin the lithium composite oxide particle may be substituted with thefluorine atoms.

In the first aspect of the present invention, the Fc may be 0 at % orhigher and 10 at % or lower. According to this configuration, highdurability and excellent input and output characteristics can becompatible with each other at a higher level.

In the above configuration, the fluorine atoms may be present in theparticle center portion in a proportion of 10 at % or lower. Accordingto this configuration, high durability and excellent input and outputcharacteristics can be compatible with each other at a higher level.

In the first aspect of the present invention, the Fs may be 30 at % orhigher. According to this configuration, high durability and excellentinput and output characteristics can be compatible with each other at ahigher level.

In the first aspect of the present invention, a difference obtained bysubtracting the Fc from the Fs may be 20 at % or greater. According tothis configuration, high durability and excellent input and outputcharacteristics can be compatible with each other at a higher level.

In the first aspect of the present invention, at least one of a portionof the nickel atoms and a portion of the manganese atoms may besubstituted with iron atoms. In addition, at least one of a portion ofthe nickel atoms and a portion of the manganese atoms may be substitutedwith titanium atoms. According to this configuration, the structuralstability of the fluorine-containing lithium nickel manganese compositeoxide can be further increased. As a result, higher durability (forexample, high-temperature cycle characteristics) or input and outputcharacteristics can be obtained.

In the first aspect of the present invention, the particle centerportion may be a region extending, toward a center of the lithiumcomposite oxide particle, from a position of 100 nm or greater inwardfrom the surface of the lithium composite oxide particle.

In the above configuration, the surface layer portion may be a region of20 nm or smaller from the surface toward the center.

In the first aspect of the present invention, the particle centerportion may be a portion of the lithium composite oxide particlecontaining no fluorine atoms and extending from a center of the lithiumcomposite oxide particle toward the surface of the lithium compositeoxide particle. In addition, the surface layer portion may be a portionof the lithium composite oxide particle containing the fluorine atomsand extending from the surface toward the center.

The second aspect of the present invention is a positive electrode for alithium-ion secondary battery including the positive active materialaccording to the first aspect of the present invention.

The third aspect of the present invention is a lithium-ion secondarybattery including the positive electrode according to the second aspectof the present invention.

The lithium-ion secondary battery in the third aspect of the presentinvention has both excellent initial characteristics and durability. Forexample, the lithium-ion secondary battery has a high energy density anda low resistance, and a reduction in the battery capacity is less likelyto occur even when high-rate charging and discharging are repeated overa long period of time. Therefore, the lithium-ion secondary battery inthe third aspect of the present invention can be appropriately used as apower source (high-output drive electric power source) for driving amotor mounted in a vehicle such as a plug-in hybrid vehicle, a hybridvehicle, and an electric vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a longitudinal sectional view schematically illustrating alithium-ion secondary battery according to an embodiment of the presentinvention; and

FIG. 2 is a TEM observation image according to Example 1, in which crossmarks (x) indicate EDX measurement points of a surface layer portion,and circles (O) indicate EDX measurement points of a particle centerportion.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed. In addition, items which are not the items (for example, theconfiguration of a positive active material) that are particularlymentioned in the specification and are necessary items (for example,other battery constituent elements that do not characterize the presentinvention, a general manufacturing process of the battery, and the like)for the implementation of the present invention can be recognized asdesign items of the skilled based on the related art in a correspondingfield. The present invention can be implemented on the basis of thecontents disclosed in the specification and general technical knowledgein the corresponding field.

In the specification, a “particle center portion” is a region includingthe center of a particle and the vicinity thereof. Typically, theparticle center portion refers to a region of about 100 nm or greaterinward from the outermost surface of the particle toward the center. Asan example, in a case where the particle is substantially spherical, theminimum circle that circumscribes the particle is drawn, and a region of200 nm or shorter from the center portion thereof, for example, 100 nmor shorter (the inner portion of a circle having a radius of 200 nm (aradius of 100 nm) from the center of the circumscribed circle) may beregarded as the particle center portion. In the specification, a“surface layer portion” refers to a region of 20 nm or smaller from theoutermost surface of a particle toward the center thereof.

The magnitude of a fluorine atom concentration at the particle centerportion or the surface layer portion can be checked as below, forexample, by energy dispersive X-ray spectroscopy (EDX) using a generaltransmission electron microscope (TEM). First, a measuring object isprepared by embedding an arbitrary fluorine-containing lithium nickelmanganese composite oxide particle in an appropriate resin and thensubjecting the resultant to cross-section polishing to expose thecross-section of the particle. In this application, the“fluorine-containing lithium nickel manganese composite oxide particle”may be regarded as a lithium composite oxide particle containing nickelatoms, manganese atoms, and fluorine atoms. Next, the cross-section isobserved through TEM at an appropriate magnification. In the obtainedTEM observation image, a particle center portion and a surface layerportion are analyzed through EDX to obtain the fluorine atomconcentrations (at %) thereof. Through EDX, atoms from boron (B) with anatomic number 5 to uranium (U) with an atomic number 92 can be detected.Therefore, using an EDX technique, the ratio of fluorine atoms (fluorineatom concentration (at %)) when the sum of atoms that belong to atomicnumbers 5 to 92 among the constituent atoms (typically, the sum ofconstituent atoms excluding lithium) is 100 at % can be calculated. Bycomparing the obtained fluorine atom concentrations, the magnitudes ofthe fluorine atom concentrations of the particle center portion and thesurface layer portion can be recognized. More appropriately, lineanalysis may be performed through EDX on a straight line from anarbitrary point positioned at the outermost surface of a particle towardthe center. By the line analysis, a change in the fluorine atomconcentration from the surface to the center of the particle can beaccurately recognized. Otherwise, the magnitude relationship between thefluorine atom concentrations can be generally recognized more simply,for example, by comparing the counts per second (CPS) of fluorine (thatis, the amount of fluorine atoms being present) in the same visual fieldof the TEM observation image.

A positive active material of an embodiment of the present inventioncontains a particulate fluorine-containing lithium nickel manganesecomposite oxide (hereinafter, may also be referred to as an “F—NiMnoxide particle”). The lithium nickel manganese composite oxide may alsobe an oxide recognized as a so-called spinel nickel manganese compositeoxide (NiMn spinel) in which a portion of a manganese site of an oxideexpressed by a general formula LiMn₂O₄ is substituted with nickel. TheF—NiMn oxide contains at least lithium, nickel, manganese, oxygen, andfluorine and may be an oxide in which a portion of oxygen atoms in thelithium nickel manganese composite oxide is replaced with fluorineatoms. By using the composite oxide as the positive active material, theoperating voltage of a lithium-ion secondary battery can be set to 4.5 Vor higher (for example, so-called 5 V class), and thus a high energydensity can be obtained.

In the embodiment of the present invention, the concentration offluorine atoms varies between the center portion of the F—NiMn oxideparticle and the surface layer portion thereof which is closer to thesurface than the particle center portion is. Specifically, the fluorineatom concentration Fe (at %) at the particle center portion and thefluorine atom concentration Fs (at %) at the surface layer portion havea relationship of Fc<Fs. For example, the fluorine atom concentrationdecreases from the outermost surface toward the center of the particlein an inclined or stepwise manner. By relatively increasing the fluorineatom concentration Fs of the surface layer portion that comes intocontact with a non-aqueous electrolyte, the binding energy of anions (O,F) and cations (Ni, Mn) at the surface layer portion can be increased.Therefore, elution of metal elements (typically manganese) from theF—NiMn oxide particle can be suppressed to a high degree. Moreover,oxygen deficiency at the surface layer portion can be compensated byfluorine, and thus the oxidative decomposition of the non-aqueouselectrolyte can be suppressed. Therefore, the amount of gas generatedduring ordinary use can be reduced. In addition, by relatively reducingthe fluorine atom concentration Fc of the particle center portion, theinteraction between fluorine and lithium ions at the particle centerportion is less likely to occur. Accordingly, Li diffusibility in theparticle can be ensured, and thus an increase in resistance can besuppressed.

The fluorine atom concentration Fs at the surface layer portion is notparticularly limited as long as it is higher than the fluorine atomconcentration Fc at the particle center portion. However, in order toexhibit the effect of the present invention at a higher level, thefluorine atom concentration Fs may be generally 1 at % or higher,typically 1.5 at % or higher, typically 10 at % or higher, and forexample, 20 at % or higher or 30 at % or higher. When the fluorine atomconcentration Fs is too high, there is a tendency to reduce the energydensity. Therefore, from this point of view, the fluorine atomconcentration Fs is typically 50 at % or lower, and preferably 40 at %or lower.

The fluorine atom concentration Fc at the particle center portion is notparticularly limited as long as it is lower than the fluorine atomconcentration Fc at the surface layer portion. For example, the particlecenter portion may contain fluorine or may not contain fluorine. Morespecifically, the fluorine atom concentration Fc is 0 at % or higher,preferably higher than 0%, more preferably 0.5 at % or higher, andtypically 1 at % or higher, and for example, may be 5 at % or higher.According to the new findings of the inventors, by allowing the particlecenter portion to contain fluorine, elution of metal elements (typicallymanganese) from the F—NiMn oxide particle can be accurately suppressed,for example, even in an aspect in which charging and discharging arerepeated under severe conditions (for example, an aspect in which ahigh-rate charging and discharging cycle is repeated in an environmentat a high temperature of about 60° C.). In addition, the fluorine atomconcentration Fc may be typically 20 at % or lower, and preferably 10 at% or lower. By minimizing the amount of fluorine atoms at the particlecenter portion, the diffusibility of lithium ions can be maintained at ahigh level, and thus the effect of the present invention can beexhibited at a higher level.

The difference (Fs−Fc) between the fluorine atom concentration Fs at thesurface layer portion and the fluorine atom concentration Fc at theparticle center portion is not particularly limited, and may betypically 1 at % or greater, for example, 2 at % or greater, typically 5at % or greater, preferably 10 at % or greater, and more preferably 20at % or greater. Otherwise, the ratio (Fc/Fs) of the Fs to the Fc maybe, for example, 2 or higher, and preferably 3 or higher. By increasingthe difference in the fluorine atom concentration between the surfacelayer portion and the center portion, fluorine atoms at the surfacelayer portion are significantly maldistributed, and thus the effect ofthe present invention can be exhibited at a higher level. On the otherhand, from the viewpoint of manufacturability (replaceability), thedifference (Fs−Fc) may be generally 50 at % or smaller, typically 40 at% or smaller, and for example, 30 at % or smaller.

Appropriate examples of the F—NiMn oxide include oxides expressed by thefollowing general formula (I).Li_(x)(Mn_(2−(a+b+c+d))Ni_(a)Ti_(b)Fe_(c)M_(d))(O_(4−y)F_(y))  (I)

In the formula (I), a, b, c, and d are real numbers that satisfy 0<a(for example, 0.4<a<0.6), 0≤b (for example, 0≤b<0.2), 0≤c (for example,0≤c<0.2), and 0≤d (for example, 0≤d<0.2). In addition, x and y are realnumbers that respectively satisfy 0.9<x<1.3 (for example, 0.9<x≤1.2),0<y<4 (typically 0<y<1, for example, 0.05≤y≤0.2, and technicallyspeaking 0<y<0.1). That is, the a represents the Ni content in theF—NiMn oxide, and more preferably satisfies 0.4<a<0.6, and even morepreferably satisfies a≤0.5. The b represents the Ti content, and fromthe viewpoint of maintaining high electron conductivity, preferablysatisfies 0.01≤b, and preferably satisfies b≤0.15. The c represents theFe content, and from the viewpoint of maintaining high battery capacity,preferably satisfies 0.01≤c, and preferably satisfies c≤0.1. The drepresents the content of an arbitrary M element, and from the viewpointof reducing resistance, preferably satisfies d≤0.07, and preferablysatisfies d≤0.05.

In addition, when 0<d is satisfied, M may be a transition metal elementor a typical metal element such as aluminum (Al), magnesium (Mg),calcium (Ca), barium (Ba), strontium (Sr), scandium (Sc), vanadium (V),chromium (Cr), cobalt (Co), copper (Cu), zinc (Zn), gallium (Ga),yttrium (Y), ruthenium (Ru), rhodium (Rh), lead (Pd), indium (In), tin(Sn), antimony (Sb), lanthanum (La), cerium (Ce), samarium (Sm),zirconium (Zr), niobium (Nb), tantalum (Ta), molybdenum (Mo), andtungsten (W). Otherwise, M may also be a metalloid element such as boron(B), carbon (C), silicon (Si), and phosphorus (P) or a non-metallicelement such as sulfur (S). One or two or more of such substitutiveelements may be employed without particular limitations.

As an example of the oxide expressed by the general formula (I), acomposite oxide which satisfies 0<b and/or 0<c, that is, contains atleast Ti and/or Fe that replaces a portion of a cation site (Ni and Mn)may be employed. For example, a fluorine-containing lithium nickelmanganese titanium composite oxide (F—LiNiFeMnTi composite oxide) whichsatisfies 0<b and 0<c, that is, contains Ti and Fe that replace aportion of a cation site (Ni and Mn) may be employed. By replacing Niand/or Mn with heterogeneous elements, higher structural stability canbe obtained, for example, even in an environment at a high temperatureof about 50° C. to 70° C. As a result, even in a case where charging anddischarging are repeated under severe conditions (for example, under theconditions of charging until the potential of the positive electrodereaches 4.5 V (vs.Li/Li⁺) in a high-temperature environment), areduction in capacity and the amount of generated gas can be suppressedto a low level. In addition, as another example of the M element, acomposite oxide containing transition metal elements other than Li, Ni,and Mn, for example, a fluorine-containing lithium nickel iron manganesetitanium cobalt composite oxide (F—LiNiFeMnTiCo composite oxide)containing Co as the M element may be employed. More specifically, forexample, LiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O_(3.95)F_(0.05) orLiNi_(0.42)Fe_(0.05)Mn_(1.45)Ti_(0.05)Co_(0.03)O_(3.95)F_(0.05) may beemployed.

Such an F—NiMn oxide particle typically has a spinel crystal structure(spinel crystal phase). In addition, for example, the F—NiMn oxideparticle may contain an F—NiMn oxide phase having the spinel crystalstructure as a primary phase and a second oxide phase in an inseparablestate. The ratio of the second oxide phase is typically lower than theratio of the spinel crystal phase. For example, when the sum of thespinel crystal phase and the second oxide phase is 100 mol %, the ratioof the second oxide phase may be 10 mol % or lower, preferably 3 mol %to 8 mol %, and for example, may be 5±1 mol %.

In an appropriate aspect, as the second oxide phase, an oxide phasehaving a layered rock salt crystal structure (layered crystal phase) andis formed of a composite oxide of lithium and transition metals isincluded. At this time, at least a portion of the interface between thespinel crystal phase and the layered crystal phase may be in a state inwhich the oxygen surfaces of the crystal structures match each other.According to the aspect, elution of transition metal elements (forexample, manganese) from the spinel crystal phase can be moreappropriately suppressed. In addition, when charging and discharging areperformed until the potential of the positive electrode reaches 4.5 V(vs.Li/Li⁺) or higher, lithium in the layered crystal phase is activatedand separated, and thus the amount of usable lithium atoms is increased.Therefore, a reduction in capacity can be suppressed to a high degree.As an appropriate example of the second oxide phase, a so-calledlithium-rich oxide expressed by, for example, a general formula Li₂MnO₃may be employed. More specifically, an oxide expressed by, for example,the following general formula (II) may be employed.Li_(3−x′)(Mn_(1−(a2+b2+c2+d2))Ni_(a2)Ti_(b2)Fe_(c2)M_(d2))_(x′)(O_(3−y′)F_(y′))  (II)

Here, in the formula (II), a2, b2, c2, and d2 are real numbers thatsatisfy 0≤a (for example, 0≤a2≤0.2), 0≤b2 (for example, 0≤b2≤0.3), 0≤c2(for example, 0≤c2≤0.2), 0≤d2 (for example, 0≤d2≤0.2), anda2+b2+c2+d2≠0. In addition, x′ and y′ are real numbers that respectivelysatisfy 1≤x′≤1.5, 0<y′<3 (typically 0<y′<1, for example, 0<y′<0.5, andtechnically speaking 0<y′<0.1). In addition, when 0<d is satisfied, M isobtained in the same manner as in the general formula (I).

The average particle diameter of F—NiMn oxide particles constituting theF—NiMn oxide may be generally 0.01 μm or greater, typically 0.1 μm orgreater, and for example, 1 μm or greater in consideration ofhandleability or workability during the manufacturing of the positiveelectrode. In addition, from the viewpoint of the homogeneous formationof a positive active material layer, the average particle diameterthereof may be generally 30 μm or smaller, typically 20 μm or smaller,and for example, 10 μm or smaller. In the specification, an “averageparticle diameter” means a particle diameter (D₅₀, also called a mediandiameter) corresponding to 50 vol. % of a cumulative frequency on thefine particle side with a small particle diameter in a particle sizedistribution in terms of volume based on a general laserdiffraction/scattering method. The shape of the F—NiMn oxide particle isnot particularly limited, and an appropriate example thereof is asubstantially spherical shape. Here, “substantially spherical shape” isa term that includes a spherical shape, a prolate spheroid shape, apolyhedral shape, and the like, and indicates, for example, a shapehaving an average aspect ratio (the ratio of the length in a major axisdirection to the length in a minor axis direction in a smallestrectangle that circumscribes a particle) of 1 to 2 (typically 1 to 1.5,for example, 1 to 1.2).

Hereinafter, a method of manufacturing the F—NiMn oxide particle inwhich the fluorine atom concentration varies between the center portionand the surface layer portion in the particle will be described.

As an appropriate aspect, a manufacturing method including a process ofpreparing a raw material hydroxide (precursor) and a baking process maybe employed. Specifically, first, the supply sources (raw material) ofmetal elements excluding Li, which are selected depending on a targetcomposition are weighed to achieve a predetermined composition ratio,and the weighed resultant is mixed with an aqueous solvent, therebypreparing an aqueous solution. As the supply sources of the metalelements excluding Li, at least manganese salt and nickel salt are used,and other metal salts (for example, titanium salt and iron salt) mayfurther be used depending on a target composition. Anions for the metalsalts may be selected so that the salts have desired water solubility.For example, sulfate ions, nitrate ions, chloride ions, carbonate ions,and the like may be selected. The anions for the metal salts may be thesame or portions thereof may be the same. Otherwise, the anions for themetal salts may be different from each other. Next, a basic aqueoussolution with a pH of 11 to 14 is added to the aqueous solution andstirred such that hydroxides including the metal elements areprecipitated by the liquid phase reaction in the aqueous solution.Accordingly, a sol-like raw material hydroxide (precursor) is obtained.As the basic aqueous solution, for example, a sodium hydroxide solutionor ammonia water may be used.

Next, the raw material hydroxide is mixed with a lithium supply sourceand a fluorine supply source, and the mixture is baked in an oxidizingatmosphere under appropriate conditions and is thereafter cooled. As thelithium supply source, Li or a compound containing Li, for example,lithium carbonate, lithium hydroxide, lithium nitrate, and lithiumacetate may be used. The amount of the mixed lithium supply source isnot particularly limited. However, in order to allow the spinel crystalphase and the layered crystal phase as the second oxide phase to bepresent while being mixed with each other, for example, Li may beadjusted to 1.1 mol or higher (typically, about 1.1 mol to 1.3 mol, forexample, about 1.2 mol) when the sum of the metal elements in the rawmaterial hydroxide is 2 mol so that Li is rich. As the fluorine supplysource, an F₂ gas or a compound containing F, for example, lithiumfluoride and ammonium fluoride may be used. The amount of the mixedfluorine supply source is not particularly limited, and may be 0.01 mol% to 0.5 mol % (for example, 0.02 mol % to 0.3 mol %) with respect tothe entirety (100 mol %) of the raw material hydroxide. By controllingthe amount of the mixed fluorine supply source, the fluorine atomconcentration (typically the amount of fluorine substituted with oxygen)can be controlled.

Here, baking is performed in multiple stages by changing the bakingtemperature during the baking. Accordingly, the F—NiMn oxide particle inwhich the fluorine atom concentration varies between the center portionand the crystal phase of the particle can be obtained. For example, thebaking may be performed in two stages in which holding for several totens of hours at about 700° C. to 900° C. is performed and thereafterholding for tens of minutes to several hours at about 900° C. to 1000°C. is further performed. By initially performing holding for asufficiently long period of time at a relatively low temperature,fluorine can be sufficiently diffused to the center portion of theparticle, and fluorine can be homogeneously contained in the particle.Thereafter, baking is performed for a relatively short time at anincreased baking temperature such that the surface layer portion of theparticle selectively contains fluorine. Accordingly, the fluorine atomconcentration of the surface layer portion can be increased. As aresult, the F—NiMn oxide particle in which the fluorine atomconcentration of the particle center portion is light and the fluorineatom concentration of the surface layer portion is dense can beobtained. That is, by controlling the baking conditions (bakingtemperature and backing time), the fluorine atom concentrations of theparticle center portion and the surface layer portion can berespectively controlled to appropriate values.

In addition, as another appropriate aspect, NiMn oxide particles whichdo not contain fluorine (oxygen is not substituted with fluorine(unsubstituted)) are obtained by purchasing a commercial product, andthe particles are exposed to a fluorine-based gas (for example, fluorinegas) under appropriate conditions (for example, an environment with areduced pressure or an environment at a high temperature). In thismethod, the surface layer portion of the particle can selectivelycontain fluorine, and thus the fluorine atom concentration of thesurface layer portion can be increased. As a result, an F—NiMn oxideparticle in which fluorine is rarely present in the particle centerportion and the fluorine atom concentration of the surface layer portionis dense.

The above-described positive active material can be used as a positiveactive material for a lithium-ion secondary battery. That is, thepositive electrode of the lithium-ion secondary battery of theembodiment of the present invention typically includes a positiveelectrode collector, and a positive active material layer which isformed on the positive electrode collector and contains the positiveactive material described above. As the positive electrode collector, aconductive member made of a metal having good conductivity (for example,aluminum) may be appropriately employed. In addition, the positiveactive material layer may contain an arbitrary component such as aconductive material or a binder as necessary in addition to the positiveactive material. As the conductive material, for example, a carbonmaterial such as carbon black (for example, acetylene black and KetjenBlack) is exemplified. As the binder, for example, polyvinylidenefluoride (PVdF) and polyethylene oxide (PEO) are exemplified.

The positive electrode of the lithium-ion secondary battery of theembodiment of the present invention may have an operating potential(vs.Li/Li⁺) of 4.5 V or higher (preferably 4.6 V or higher, and morepreferably 4.7 V) in a range of an SOC value of 0% to 100%. By allowingthe operating potential of the positive electrode to be 4.5 V or higher,the potential difference (voltage) between the positive and negativeelectrodes can be increased, and thus a high energy density of thebattery can be obtained. In general, the operating potential between 0%and 100% of the SOC value is maximized when the SOC value is 100%.Therefore, typically, the operating potential of the positive electrodecan be recognized (for example, whether or not the operating potentialis 4.5 V or higher) using the operating potential of the positiveelectrode when the SOC value is 100% (that is, fully charged state).

In addition, the operating potential of the positive electrode in arange of an SOC value of 100%, for example, measurement can be performedas follows. First, a positive electrode as a measurement object isprepared, and a three electrode cell is constructed by using thepositive electrode as a working electrode (WE), using metal lithium as acounter electrode (CE), and metal lithium as a reference electrode (RE).Next, the SOC of the three electrode cell is adjusted to 100% on thebasis of the theoretical capacity of the three electrode cell. Theadjustment of the SOC can be performed by a charging process between theWE and the CE using, for example, a general charging and dischargingdevice or a potentiostat. By measuring the potential between the WE andthe RE of the three electrode cell having an adjusted SOC, the potentialcan be determined as the operating potential (vs.Li/Li⁺) of the positiveelectrode in the SOC state.

The lithium-ion secondary battery can be manufactured by using thepositive electrode, a negative electrode, and a non-aqueous electrolyte.The negative electrode typically includes, similarly to the positiveelectrode, a negative electrode collector, and a negative activematerial layer formed on the negative electrode collector. The negativeactive material may contain an arbitrary component (for example, abinder or a thickener) in addition to the negative active material. Asthe negative electrode collector, a conductive material made of a metalhaving good conductivity (for example, copper) may be appropriatelyemployed. As the negative active material, for example, a carbonmaterial such as graphite may be employed. As the binder,styrene-butadiene rubber (SBR) or the like may be employed. As thethickener, for example, carboxymethyl cellulose (CMC) or the like may beemployed. As the non-aqueous electrolyte, an electrolyte containing asupport salt in a non-aqueous solution (non-aqueous electrolyticsolution) may be appropriately used. As the support salt, for example,LiPF₆ and LiBF₄ may be employed. As an organic solvent, for example, anaprotic solvent such as carbonates, esters, and ethers may be employed.Among these, a solvent having high oxidation resistance (that is, a highoxidative decomposition potential), for example, a fluorinated cycliccarbonate such as monofluoroethylene carbonate (MFEC) or a fluorinatedchain carbonate such as (2,2,2-trifluoroethyl)methyl carbonate (F-DMC)may be appropriately employed.

Although not intended for particular limitation, as an embodiment of thepresent invention, a lithium-ion secondary battery having a form inwhich a flat wound electrode assembly and a non-aqueous electrolyte areaccommodated in a battery case having a flat rectangular parallelepipedshape is exemplified. In the following drawings, like members and siteshaving the same actions are denoted by like reference numerals, and anoverlapping description may be omitted or simplified. In each of thedrawings, the dimensional relationships (length, width, thickness, andthe like) do not necessarily reflect actual dimensional relationships.

FIG. 1 is a longitudinal sectional view schematically illustrating thecross-sectional structure of a lithium-ion secondary battery 100. In thelithium-ion secondary battery 100, an electrode assembly (woundelectrode assembly) 80 having a form in which a long positive electrodesheet 10 and a long negative electrode sheet 20 are wound in a flatmanner via a long separator sheet 40, and a non-aqueous electrolyte areaccommodated in a battery case 50 having a flat box shape.

The battery case 50 includes a battery case main body 52 having a flatrectangular parallelepiped shape (box shape) with an open upper end, anda cover 54 which blocks the opening. In the upper surface (that is, thecover 54) of the battery case 50, a positive electrode terminal 70 forexternal connection, which is electrically connected to the positiveelectrode of the wound electrode assembly 80, and a negative electrodeterminal 72 which is electrically connected to the negative electrode ofthe wound electrode assembly 80 are provided. Like the battery case of alithium-ion secondary battery in the related art, the cover 54 is alsoprovided with a safety valve 55 for discharging gas generated in thebattery case 50 to the outside of the case 50.

In the battery case 50, the flat wound electrode assembly 80 and thenon-aqueous electrolyte (not illustrated) are accommodated. The woundelectrode assembly 80 has a long sheet structure in a previous stageduring assembly. The positive electrode sheet 10 includes a longpositive electrode collector, and a positive active material layer 14formed on at least one surface (typically both surfaces) of the positiveelectrode collector along the longitudinal direction thereof. Thenegative electrode sheet 20 includes a long negative electrodecollector, and a negative active material layer 24 formed on at leastone surface (typically both surfaces) of the negative electrodecollector along the longitudinal direction thereof. In addition, betweenthe positive active material layer 14 and the negative active materiallayer 24, two long sheet-like separators (separator sheets) 40 aredisposed as an insulating layer for preventing direct contact betweenthe two layers. As the separator, a porous resin sheet formed from aresin such as polyethylene (PE) and polypropylene (PP) may beappropriately used.

In a width direction specified as a direction from one end portion ofthe wound electrode assembly 80 in the winding axis direction thereof tothe other end portion thereof, the center portion thereof is providedwith a wound core portion formed by overlapping and densely laminatingthe positive active material layer 14 formed on the surface of thepositive electrode collector and the negative active material layer 24formed on the surface of the negative electrode collector. In addition,in both end portions of the wound electrode assembly 80 in the windingaxis direction thereof, a positive active material layer non-formationportion of the positive electrode sheet 10 and a negative activematerial layer non-formation portion of the negative electrode sheet 20protrude outward from the wound core portion. In addition, a positiveelectrode collector plate is attached to a positive electrode sideprotrusion portion (the positive active material layer non-formationportion), and a negative electrode collector plate is attached to anegative electrode side protrusion portion (the negative active materiallayer non-formation portion) to be electrically connected to theabove-mentioned positive electrode terminal 70 and the negativeelectrode terminal 72, respectively.

The lithium-ion secondary battery manufactured according to themanufacturing method of the embodiment of the present invention can beused for various applications and also exhibits a high energy density,excellent input and output characteristics, and high durability.Therefore, by emphasizing the characteristics, the lithium-ion secondarybattery can be preferably used for an application that requires a highenergy density, a high input and output density, and high durability, oran application in which the use environment is at a high temperature of50° C. or higher. Examples of the applications include a power sourcefor a motor mounted in a vehicle (drive power source). The type of thevehicle is not particularly limited, and typically, a vehicle, forexample, a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), anelectric vehicle (EV), and the like may be employed. The lithium-ionsecondary battery may be typically used in a battery pack form in whicha plurality of lithium-ion secondary batteries are connected in seriesand/or in parallel.

Hereinafter, several examples regarding the present invention will bedescribed. However, it is not intended to limit the present invention tothe examples.

The production of a positive active materialLiNi_(0.5)Mn_(1.5)O_(3.98)F_(0.02)—Li₂MnO_(2.99)F_(0.01) of ComparativeExample 1 will be described below. First, nickel sulfate (NiSO₄) andmanganese sulfate (MnSO₄) were dissolved in water to achieve the abovecomposition, and sodium hydroxide (NaOH) was added thereto and stirredwhile being neutralized, thereby obtaining a raw material hydroxideaccording to Comparative Example 1. The raw material hydroxide was mixedwith lithium carbonate (Li₂CO₃) and 0.02 mol % of lithium fluoride (LiF)with respect to the entirety (100 mol %) of the raw material hydroxide,and the mixture was baked at 900° C. in an air atmosphere for 15 hoursand was crushed by a ball mill, thereby obtaining a fluorine-containinglithium nickel manganese composite oxide having an average particlediameter of 5 μm. In addition, the X-ray diffraction profile of aprimary particle included in the composite oxide was analyzed on thebasis of the Rietveld method. As a result, it was confirmed that a firstphase (primary phase) made of LiNi_(0.5)Mn_(1.5)O_(3.98)F_(0.02), and asecond phase which had a layered crystal structure and was made ofLi₂MnO_(2.99)F_(0.01) were included. In addition, it was confirmed thatthe molar ratio of the phases (the first phase: the second phase) was0.95:0.05 (that is, when the total amount of the positive activematerial was 100 mol %, the ratio of Li₂MnO_(2.99)F_(0.01) was 5 mol %).

The production of a positive active materialLiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05)—Li₂MnO_(2.96)F_(0.04) of ComparativeExample 2 will be described below. In Comparative Example 2, afluorine-containing lithium nickel manganese composite oxide having anaverage particle diameter of 5 μm was obtained in the same manner asthat of Comparative Example 1 except that 0.1 mol % of lithium fluoride(LiF) with respect to the entirety (100 mol %) of the raw materialhydroxide was added during baking.

The production of a positive active materialLiNi_(0.5)Mn_(1.5)O_(3.9)F_(0.1)—Li₂MnO_(2.93)F_(0.07) of ComparativeExample 3 will be described below. In Comparative Example 3, afluorine-containing lithium nickel manganese composite oxide having anaverage particle diameter of 5 μm was obtained in the same manner asthat of Comparative Example 1 except that 0.3 mol % of lithium fluoride(LiF) with respect to the entirety (100 mol %) of the raw materialhydroxide was added during baking.

The production of a positive active materialLiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05)—Li₂MnO_(2.96)F_(0.04) of Example 1will be described below. First, nickel sulfate (NiSO₄) and manganesesulfate (MnSO₄) were dissolved in water to achieve the abovecomposition, and sodium hydroxide (NaOH) was added thereto and stirredwhile being neutralized, thereby obtaining a raw material hydroxideaccording to Example 1. The resultant was mixed with a predeterminedamount of lithium carbonate (Li₂CO₃), and the mixture was baked at 900°C. in an air atmosphere for 15 hours and was crushed by a ball mill,thereby obtaining a lithium nickel manganese composite oxide(LiNi_(0.5)Mn_(1.5)O₄—Li₂MnO₃) having an average particle diameter of 5μm (fluorine was not contained). The composite oxide was exposed to anF₂ gas atmosphere in a sealed container under an environment at 25° C.,thereby obtaining a fluorine-containing lithium nickel manganesecomposite oxide.

The production of a positive active materialLiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05)—Li₂MnO_(2.96)F_(0.04) of Example 2will be described below. A fluorine-containing lithium nickel manganesecomposite oxide having an average particle diameter of 5 μm was obtainedin the same manner as that of Comparative Example 2 except that the rawmaterial hydroxide was mixed with a predetermined amount of lithiumcarbonate (Li₂CO₃) and lithium fluoride (LiF) and baking was performedin two stages in which the mixture was held at 900° C. in an airatmosphere for 15 hours and was thereafter held at 930° C. for 30minutes.

The production of a positive active materialLiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O_(3.95)F_(0.05)—Li₂MnO_(2.96)F_(0.04)of Comparative Example 4 will be described below. A fluorine-containinglithium nickel iron manganese titanium composite oxide having an averageparticle diameter of 5 μm was obtained in the same manner as that ofComparative Example 2 except that the nickel sulfate (NiSO₄), ironsulfate (FeSO₄), manganese sulfate (MnSO₄), and titanium sulfate (TiSO₄)were dissolved in water to achieve the above composition, and sodiumhydroxide (NaOH) was added thereto and stirred while being neutralizedto obtain a raw material hydroxide.

The production of a positive active materialLiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O_(3.95)F_(0.05)—Li₂MnO_(2.96)F_(0.04)of Example 3 will be described below. A fluorine-containing lithiumnickel iron manganese titanium composite oxide having an averageparticle diameter of 5 μm was obtained in the same manner as that ofExample 2 except that the nickel sulfate (NiSO₄), iron sulfate (FeSO₄),manganese sulfate (MnSO₄), and titanium sulfate (TiSO₄) were dissolvedin water to achieve the above composition, and sodium hydroxide (NaOH)was added thereto and stirred while being neutralized to obtain a rawmaterial hydroxide. The properties of the positive active materials arecollected in Table 1 below.

Hereinafter, the distribution of the fluorine atom concentration will bedescribed. The obtained composite oxide particle was subjected toembedding and polishing to expose the cross-section of the particle, andthe cross-section was observed with a transmission electron microscope(TEM). For the obtained TEM observation image, the fluorine atomconcentrations (at %) at the particle center portion and the surfacelayer portion were obtained using energy dispersive X-ray spectroscopy(EDX). As an example, in FIG. 2, a TEM observation image of Example 1 isillustrated. Measurement of the particle center portion was performed onthree points indicated by circles (O) in FIG. 2. Specifically, thesmallest circle that circumscribes the particle was drawn on thecross-sectional TEM observation image of the particle, and arbitrarythree points were selected from a portion of the spinel crystal phase ina region within 200 nm from the center thereof (a region within a radiusof 200 nm from the center of the circumscribed circle) and weresubjected to line analysis. An arithmetic average of the obtainedresults was obtained and determined as the fluorine atom concentrationFc (at %) of the particle center portion. In addition, measurement ofthe surface layer portion was performed on three points indicated bycross marks (x) in FIG. 2. Specifically, arbitrary three points wereselected from a portion of the spinel crystal phase in a region within20 nm from the outermost surface of the particle toward the centerthereof on the cross-sectional TEM observation image of the particle andwere subjected to line analysis. An arithmetic average of the obtainedresults was obtained and determined as the fluorine atom concentrationFs (at %) of the surface layer portion. In addition, the portion of thespinel crystal phase is distinguished from portions made of othercrystal phases (for example, the layered crystal phase) by, for example,gray scale of the TEM observation image or electron beam diffraction andthus can be recognized. The fluorine atom concentrations (at %) of thecenter portion and the outermost surface portion of the particleaccording to each of the examples are shown in Table 1.

TABLE 1 High-temperature cycle test (60° C., 2 C.) Fluorine atomDeterioration concentration (atm %) capacity Internal Surface Particle(relative value) Amount of DC Specification of positive active materiallayer center Initial Latter generated resistance Experimental Molarportion portion Distri- period period gas (relative (relative ExamplesComposition ratio Fs Fc bution 30 cyc. 200 cyc. value) value)Comparative First LiNi_(0.5)Mn_(1.5)O_(3.98)F_(0.02) 95 mol % 6 6 Absent25 100 100 100 Example 1 phase Second Li₂MnO_(2.99)F_(0.01) 5 mol %phase Comparative First LiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05) 95 mol % 1010 Absent 22 84 85 103 Example 2 phase Second Li₂MnO_(2.96)F_(0.04) 5mol % phase Comparative First LiNi_(0.5)Mn_(1.5)O_(3.9)F_(0.1) 95 mol %30 30 Absent 19 76 75 122 Example 3 phase Second Li₂MnO_(2.93)F_(0.07) 5mol % phase Example 1 First LiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05) 95 mol %30 0 Present 19 81 77 101 phase Second Li₂MnO_(2.96)F_(0.04) 5 mol %phase Example 2 First LiNi_(0.5)Mn_(1.5)O_(3.95)F_(0.05) 95 mol % 30 10Present 19 78 76 103 phase Second Li₂MnO_(2.96)F_(0.04) 5 mol % phaseComparative First LiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O_(3.95)F_(0.05)95 mol % 10 10 Absent 11 48 53 87 Example 4 phase SecondLi₂MnO_(2.96)F_(0.04) 5 mol % phase Example 3 FirstLiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O_(3.95)F_(0.05) 95 mol % 30 10Present 8 33 35 85 phase Second Li₂MnO_(2.96)F_(0.04) 5 mol % phase

As shown in Table 1, in Comparative Examples 1 to 4, the fluorine atomconcentration in the particle was substantially homogeneous. It isthought that this is because fluorine could be homogeneously dispersedto the center portion of the particle by holding the composite oxide ata constant baking temperature for a sufficiently long period of time. InExample 1, since the composite oxide that did not contain fluorine wasused, fluorine was rarely present in the particle center portion, andfluorine was present only in the surface layer portion (outermostsurface) of the particle. In Examples 2 and 3, the fluorine atomconcentration of the particle center portion was light, and the fluorineatom concentration of the surface layer portion was dense. That is,baking was performed in two stages in which holding was performed at afirst stage at a holding temperature for a relatively long period oftime and thereafter holding was performed at a second stage at a higherholding temperature than that of the first stage for a relatively shorttime. Accordingly, the fluorine atom concentration in the particle wasinclined.

Hereinafter, the production of the positive electrode sheet will bedescribed. The positive active material produced above, acetylene blackas a conductive material, and polyvinylidene fluoride (PVdF) as a binderwere weighed to achieve a material mass ratio of 87:10:3, and were mixedin N-Methyl-2-pyrrolidone (NMP), thereby preparing a slurry-likecomposition for forming a positive active material layer. Thecomposition was applied to an aluminum foil (positive electrodecollector) having a thickness of 15 μm and was dried and pressed,thereby producing a positive electrode sheet (Comparative Examples 1 to4 and Examples 1 to 3) in which the positive active material layer wasformed on the positive electrode collector.

Hereinafter, the production of the negative electrode sheet will bedescribed. A natural graphite material (with an average particlediameter of 20 μm) as the negative active material, a styrene-butadienecopolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as athickener were weighed to achieve a material mass ratio of 98:1:1, andwere mixed in water, thereby preparing a slurry-like composition forforming a negative active material layer. The composition was applied toa copper foil (negative electrode collector) having a thickness of 10 μmand was dried and pressed, thereby producing a negative electrode sheetin which the negative active material layer was formed on the negativeelectrode collector.

Hereinafter, the production of the lithium-ion secondary battery will bedescribed. As the non-aqueous electrolyte, an electrolyte obtained bydissolving LiPF₆ as a support salt at a concentration of 1.0 mol/L in amixed solvent containing monofluoroethylene carbonate (MFEC) as afluorinated cyclic carbonate and (2,2,2-trifluoroethyl)methyl carbonate(F-DMC) as a fluorinated chain carbonate at a volume ratio ofMFEC:F-DMC=50:50 was prepared. The positive electrode sheet and thenegative electrode sheet produced as described above were faced oppositeto each other with a separator sheet (here, a composite porous sheet inwhich polypropylene layers were laminated on both sides of apolyethylene layer was used) interposed therebetween, thereby producingan electrode assembly. The electrode assembly and the non-aqueouselectrolyte were accommodated in a laminate case and were sealed,thereby producing a lithium-ion secondary battery (laminate cell).

Hereinafter, an activation process and the measurement of an initialcapacity will be described. The lithium-ion secondary battery producedas described above was charged with a constant current (CC) at a rate of1/5 C in an environment at a temperature of 25° C. until the voltagereaches 4.9 V, and thereafter constant voltage (CV) charging wasperformed until the current value reaches 1/50C. This state wasspecified as a fully charged state (SOC100%). Thereafter, constantcurrent (CC) discharging was performed at a rate of 1/5 C in anenvironment at a temperature of 25° C. until the voltage reaches 3.5 V.The CC discharge capacity at this time was specified as an initialcapacity. Here, 1 C means a current value with which a battery capacity(Ah) predicted from the theoretical capacity of the positive electrodecan be charged within one hour.

Hereinafter, the measurement of DC resistance will be described. Eachlithium-ion secondary battery was subjected to CC charging at a rate of1/5 C in an environment at a temperature of 25° C. until the SOC reaches50% when the initial capacity is specified as 100%. A discharge pulsecurrent was applied to the battery that was adjusted to an SOC 50%state, at a rate of 5 C for ten seconds. In addition, a voltage dropvalue (V) for ten seconds was divided by the corresponding current value(V), thereby calculating an internal DC resistance. The results areshown in the “internal DC resistance” row in Table 1. In Table 1,relative values are shown when the internal DC resistance of thelithium-ion secondary battery according to Comparative Example 1 isspecified as the reference (100).

Hereinafter, a high-temperature cycle test will be described. Chargingand discharging of each lithium-ion secondary battery were repeated in200 cycles under a temperature condition of 60° C., and thereafter thebattery capacity (CC discharge capacity) after each cycle was measuredin the same manner as the initial capacity measurement. In addition, asfor charging and discharging conditions for one cycle during thehigh-temperature cycle test, after CC charging was performed at acharging rate of 2 C until the voltage reaches 4.9 V, CC discharging wasperformed at a discharging rate of 2 C until the voltage reaches 3.5 V.In addition, the battery capacity after N cycles was subtracted from theinitial capacity, thereby obtaining a deterioration capacity after eachcycle. As a representative value, results after 30 cycles in the initialperiod of the charging and discharging cycles, and results after 200cycles in the latter period of the charging and discharging cycles areshown in the corresponding rows of Table 1. In addition, in Table 1,relative values are shown when the deterioration capacity of thelithium-ion secondary battery according to Comparative Example 1 isspecified as the reference (100).

The evaluation of the amount of generated gas was performed by using theArchimedes method. That is, first, the lithium-ion secondary batteryimmediately after being produced was immersed in a container filled witha fluorine-based inert liquid (Fluorinert (trademark) made by Sumitomo3M. Limited), and the initial volume was measured from a change inweight before and after the immersion. Furthermore, the volume wasmeasured in the same method after the high-temperature cycle test, and avalue obtained by subtracting the initial volume from the volume afterthe cycle test was calculated as the amount of generated gas. Theresults are shown in the corresponding rows of Table 1. In addition, inTable 1, relative values are shown when the amount of gas generated inthe lithium-ion secondary battery according to Comparative Example 1 isspecified as the reference (100).

As shown in Table 1, in Comparative Examples 1 to 3 in which thefluorine atom concentration in the composite oxide particle issubstantially homogeneous, the deterioration capacity and the amount ofgenerated gas after the high-temperature cycle test were reduced as theamount of added fluorine is increased. The reduction in thedeterioration capacity is an effect caused by a situation in which bondbetween the transition metals (particularly Mn) that are easily elutedas the amount of added fluorine is increased and fluorine (F) becomesstronger and thus the elution of the transition metals is suppressed. Inaddition, the reduction in the amount of generated gas is an effectcaused by a situation in which fluorine compensates for oxygen-deficientportions of the surface of the particle which act as the origin of gasgeneration (decomposition of the non-aqueous electrolyte) as the amountof added fluorine is increased are compensated, to a high degree. On theother hand, the internal DC resistance value was substantially the samein Comparative Examples 1 and 2 in which the amount of added fluorinewas relatively small and was significantly increased in ComparativeExample 3 in which the amount of added fluorine was relatively large. Itis estimated that the increase in the resistance is a result caused by asituation in which the interaction between fluorine that was present inthe particle and the charge carriers (lithium) was increased as theamount of added fluorine is increased and thus the diffusibility(mobility) of lithium in the particle was degraded.

In Example 1, the internal DC resistance was suppressed at an equallevel to that of Comparative Examples 1 and 2. It is thought that thisis because fluorine was rarely present in the particle center portionand thus the diffusibility (mobility) of lithium in the particle wasensured. In addition, the deterioration capacity and the amount ofgenerated gas were suppressed to a low level between ComparativeExamples 2 and 3. It is thought that this is because the fluorine atomconcentration of the surface layer portion of the particle in Example 1was dense (the oxygen deficiency amount of the particle surface wassmall). As described above, it was seen that in Example 1, thesuppression of capacity deterioration and gas generation afterhigh-temperature cycles and a reduction in resistance can be compatiblewith each other.

In addition, in Example 2 in which the fluorine atom concentration ofthe particle center portion was light and the fluorine atomconcentration of the surface layer portion was dense, the internal DCresistance was suppressed to a low level similarly to Example 1.Moreover, the deterioration capacity and the amount of generated gaswere further reduced than those of Example 1 and were equal to that inComparative Example 3 in which the amount of added fluorine was large.As described above, it was seen that in Example 2, capacitydeterioration and gas generation could be further suppressed whilemaintaining a low resistance. Particularly in Example 2, thedeterioration capacity in the latter period of the charging anddischarging cycles (after 200 cycles) was improved compared toExample 1. It is thought that this is because the elution of thetransition metals in the latter period of the charging and dischargingcycles could be suppressed by including fluorine in the particle centerportion. That is, it could be seen that since fluorine was present inthe particle (here, the particle center portion) in a low proportion(for example, 10 at % or higher), capacity deterioration in the latterperiod of the charging and discharging cycles could be suppressed.

In addition, in Example 3 in which a portion of Mn and Ni wassubstituted with Fe and Ti, the deterioration capacity, the amount ofgenerated gas, and the internal DC resistance were further reduced thanthose of Example 2. It is thought that this is an effect caused by asituation in which, when fluorine was contained in the composite oxideparticle, the average valence of manganese was increased from +3 to +4in order to maintain the charge balance and thus the ratio of stableMn⁴⁺ was increased, or fluorine compensated for oxygen-deficientportions and thus the oxygen deficiency amount was reduced. As describedabove, by using the positive active material of Examples of the presentinvention, a lithium-ion secondary battery having both high durability(for example, high-temperature high-rate cycle characteristics) andexcellent input and output characteristics can be obtained.

While the present invention has been described in detail, theembodiments and Examples are merely examples, and various modificationsand changes of the specific examples described above are included in thepresent invention.

What is claimed is:
 1. A positive active material for a lithium-ionsecondary battery, the positive active material comprising: a lithiumcomposite oxide particle represented by the following general formula(I):Li_(x)(Mn_(2−(a+b+c+d))Ni_(a)Ti_(b)Fe_(c)M_(d))(O_(4−y)F_(y))  (I)wherein 0.4<a<0.5, 0.01≤b<0.2, 0.01≤c<0.1, 0≤d<0.2, 0.9<x<1.3,0.05≤y≤0.2, and when 0<d is satisfied, M is at least one elementselected from the group consisting of aluminum, magnesium, calcium,barium, strontium, scandium, vanadium, chromium, cobalt, copper, zinc,gallium, yttrium, ruthenium, rhodium, lead, indium, tin, antimony,lanthanum, cerium, samarium, zirconium, niobium, tantalum, molybdenum,and tungsten, the lithium composite oxide particle contains a spinelcrystal phase as a first oxide phase and a layered crystal phase as asecond oxide phase, the lithium composite oxide particle includes aparticle center portion and a surface layer portion that is closer to asurface of the lithium composite oxide particle than the particle centerportion is, and a fluorine atom concentration Fc (at %) of the particlecenter portion measured by energy dispersive X-ray spectroscopy is lowerthan a fluorine atom concentration Fs (at %) of the surface layerportion.
 2. The positive active material according to claim 1, wherein aportion of oxygen atoms in the lithium composite oxide particle issubstituted with the fluorine atoms.
 3. The positive active materialaccording to claim 2, wherein the fluorine atoms are present in theparticle center portion in a proportion of 10 at % or lower.
 4. Thepositive active material according to claim 1, wherein the Fc is 0 at %or higher and 10 at % or lower.
 5. The positive active materialaccording to claim 1, wherein the Fs is 30 at % or higher.
 6. Thepositive active material according to claim 1, wherein a differenceobtained by subtracting the Fc from the Fs is 20 at % or greater.
 7. Thepositive active material according to claim 1, wherein at least one of aportion of the nickel atoms or a portion of the manganese atoms issubstituted with iron atoms, and at least one of a portion of the nickelatoms or a portion of the manganese atoms is substituted with titaniumatoms.
 8. The positive active material according to claim 1, wherein theparticle center portion is a region extending, toward a center of thelithium composite oxide particle, from a position of 100 nm or greaterinward from the surface of the lithium composite oxide particle.
 9. Thepositive active material according to claim 8, wherein the surface layerportion is a region of 20 nm or smaller from the surface toward thecenter.
 10. The positive active material according to claim 1, whereinthe particle center portion is a portion of the lithium composite oxideparticle containing no fluorine atoms and extending from a center of thelithium composite oxide particle toward the surface of the lithiumcomposite oxide particle, and the surface layer portion is a portion ofthe lithium composite oxide particle containing the fluorine atoms andextending from the surface toward the center.
 11. A positive electrodefor a lithium-ion secondary battery, the positive electrode comprisingthe positive active material according to claim
 1. 12. A lithium-ionsecondary battery comprising the positive electrode according to claim11.